1. Field of the Invention
The present invention relates generally to transmission of optical signals in a communication network and, more particularly, to reflection suppression structures in semiconductor circuits, such as photonic integrated circuits, as part of such communication networks.
2. Description of the Related Art
Optical telecommunication networks have become increasingly more advanced as the need for high speed optical traffic increases. Some of the primary functions carried out in an optical telecommunication network include the amplification of a wavelength division multiplexed (WDM) signal comprising a plurality of individual optical channel signals, providing the optical channels signals for further processing. One way this need for higher speed throughput has been addressed is through the use of semiconductor integrated circuits, more specifically, photonic integrated circuits (PICs). These devices provide the integration of both active and passive optical components on a single substrate and are integrated with other optical components to form a multifunctional optical device. Compared to the deployment of discrete optical components, such monolithic PIC chips can significantly reduce the size of optical components necessary in the optical system, as well as significantly reducing the overall costs of such systems. Such PIC chips can be fabricated to perform as a receiver photonic integrated circuit (RxPIC), a transmitter photonic integrated circuit (TxPIC), or both.
Generally, RxPIC systems, among other things, comprise an optical amplifier, such as a semiconductor optical amplifier (SOA), an arrayed waveguide grating (AWG) and a plurality of light detectors, such as photodiodes (PD). The SOA has an input which accepts a multiplexed optical signal, from an optical link for example, and amplifies the signal prior to passing it on to an input of the AWG. The AWG then demultiplexes the incoming multiplexed optical signal into a plurality of channel signals, each of the plurality of channel signals are provided as an output from the AWG and coupled to a respective photodiode for additional processing of the signal channel.
The semiconductor optical amplifier is advantageous in such PIC structures since it has the ability to directly amplify the incoming optical signal without the need to first convert the incoming optical signal into the electrical domain. The SOA is essentially a laser diode, having an optical gain region formed between two end facets. The end facets of the SOA typically are provided with an anti-reflective material to prevent light from being reflected back into the gain medium of the SOA, ultimately preventing the amplifier from lasing. Aside from anti-reflective coatings, other measures can be performed to reduce reflections which may cause the SOA to operate as a laser. For example, the input and output facets of the gain medium may be angled with respect to a longitudinal axis of the gain medium reducing further the amount of amplified light reflected back into the gain medium. However, this leads to alignment problems when coupling the amplified signal to other optical components.
When an SOA is integrated into a monolithic semiconductor device, such as a PIC device, however, other optical components and the interfaces or transitions between the optical components, as well as the input and output facets of such a monolithic device, may provide a source of undesirable reflections. For example, the various interface points or transition points between the free space regions and adjacent waveguide structures of the AWG may undesirably reflect light back into the device and toward the SOA, where the reflected signal is amplified once again. Where multiple reflective surfaces exist, optical energy is repeatably passed through the SOA, the optical energy subsequently amplified during each pass through the SOA, eventually, reaching a lasing threshold resulting in the SOA lasing at the frequency corresponding to the optical noise being reflected. While not necessarily detrimental to the signal output from the PIC device, as will be better understood with reference to the further discussion below, when lasing the SOA transforms into a gain-clamped amplifier, greatly reducing the gain achievable in the device.
What is needed is a semiconductor photonic integrated circuit which includes a plurality of optical components, at least one of the optical components including structures which greatly reduce reflections to provide a monolithic structure including integrated amplification.